Analysis of surface-emitting second-order distributed feedback lasers with central grating phaseshift

An analysis of 2nd-order distributed feedback lasers (DFB) with central grating phaseshift is performed. The devices have an active grating (ie, DFB) section, passive grating sections (ie, DBRs); and the active grating is formed at a metal-semiconductor interface. Coupled-mode theory and the transfer matrix method are employed. It is found that a central grating phaseshift, Δϕ, of 180° causes the laser to radiate in a beam of symmetric near-field amplitude profile, in sharp contrast to conventional second-order DFB lasers which radiate in beams of asymmetric near-field amplitude profile. In turn the far-field profile becomes a single-lobe beam pattern. Thus, a means to fundamentally obtain surface emission in an orthonormal single-lobe beam from a 2nd-order DFB/DBR device has been found. The orthornomal-beam emission is achieved at no penalty in device efficiency. External differential quantum efficiencies, η
_d
, in excess of 70% can be obtained, and the guided-field intensity profile is substantially uniform. The effects of the lengths of the DFB section (L
_DFB
) and of each of the DBR sections (L
_DBR
) on device performance are analyzed and optimal values are found to occur for L
_DFB
in the 500-700 μm range and for L
_DBR
in the 600-700 μm range. One can obtain η
_D
values as high as 76% from devices with 80% of the energy in the central lobe, and moderate threshold gains (ie, 40 cm
^-1
). Threshold gains as low as 25 cm
^-1
can also be obtained from highly efficient devices (ie, η
_D
≅70%), at some penalty in guided-field uniformity. In either case the intermodal discrimination is quite high (70-75 cm
^-1
). Gratings with half-wave (ie, π) phaseshifts have been fabricated by using the dual-tone photoresist method, and the concept has been experimentally proven: orthonormal, single-lobe emission in a diffraction-limited beam from 1500 μm-long devices. Extension to two-dimensional (2-D) large-aperture: 200 μm×1500 μm; surface emitters is quite possible, which should allow for the emission of watts of coherent CW power in a stable, single mode. The 2-D structure represents a defect-free, second-order active photonic lattice.